text: Making the plasma from argon, instead of other gases, has several advantages.  First, argon is abundant (in the atmosphere, as a result of the radioactive decay of potassium) and therefore cheaper than other noble gases. Argon also has a higher first ionization potential than all other elements except He, F, and Ne.  Because of this high ionization energy, the reaction  (Ar + e → Ar) is more energetically favorable than the reaction (M + e → M).  This ensures that the sample remains ionized (as M) so that the mass spectrometer can detect it.
  Argon can be purchased for use with the ICP-MS in either a refrigerated liquid or a gas form.  However it is important to note that whichever form of argon purchased, it should have a guaranteed purity of 99.9% Argon at a minimum.  It is important to determine which type of argon will be best suited for the specific situation.  Liquid argon is typically cheaper and can be stored in a greater quantity as opposed to the gas form, which is more expensive and takes up more tank space.  If the instrument is in an environment where it gets infrequent use, then buying argon in the gas state will be most appropriate as it will be more than enough to suit smaller run times and gas in the cylinder will remain stable for longer periods of time, whereas liquid argon will suffer loss to the environment due to venting of the tank when stored over extended time frames.  However, if the ICP-MS is to be used routinely and is on and running for eight or more hours each day for several days a week, then going with liquid argon will be the most suitable.  If there are to be multiple ICP-MS instruments running for long periods of time, then it will most likely be beneficial for the laboratory to install a bulk or micro bulk argon tank which will be maintained by a gas supply company, thus eliminating the need to change out tanks frequently as well as minimizing loss of argon that is left over in each used tank as well as down time for tank changeover.
  Helium can be used either in place of, or mixed with, argon for plasma generation. Helium's higher first ionisation energy allows greater ionisation and therefore higher sensitivity for hard-to-ionise elements. The use of pure helium also avoids argon-based interferences such as ArO. However, many of the interferences can be mitigated by use of a collision cell, and the greater cost of helium has prevented its use in commercial ICP-MS.